In optoelectronics systems, semiconductor lasers will often experience changes in wavelength due to aging and temperature variations. This shifting of the wavelength is particularly critical in dense wavelength division multiplexing applications where different channels are separated by a small wavelength difference, typically corresponding to an optical frequency difference of 50-500 Ghz. There is often a need to closely monitor the wavelength, and if it is drifting away from its desired value, to supply an error signal that is fed back to a wavelength control.
A standard approach to this monitoring function would be to use two tapped optical signals passed through two optical filters and detected by two photodetectors. The two resulting electrical signals arc passed to a balanced, differential comparator to produce the error signal. Such a technique is suitable as long as the two optically tapped signals, the optical filters, and the two photo-detectors all remain stable, and this is not always the case.